If we have a mixture of:
- a MariaDB standard (built-in permanent) data type, and
- a non-standard (optionally compiled or pluggable) data type,
then ask the type collection of the non-standard type to aggregate the mixture.
If the non-standard collection fails, then continue aggregation
with Type_handler_data.
Consider an IN predicate with ROW-type arguments:
predicant IN (value1, ..., valueM)
where predicant and all values consist of N elements.
When performing IN for these arguments, at every position i (1..N)
only data type of i-th element of predicant was taken into account,
while data types on i-th elements of value1..valueM were not taken.
These led to bad comparison data type detection, e.g. when
mixing unsigned and signed integer values.
After this change all element data types are taken into account.
So, for example, a mixture of unsigned and signed values is
now calculated using decimal and does not overflow any more.
Detailed changes:
1. All comparators for ROW elements are now created recursively
at fix_fields() time, inside cmp_item_row::prepare_comparators().
Previously prepare_comparators() installed comparators only
for temporal data types, while comparators for other types were
installed at execution time, in cmp_item_row::store_value().
2. Removing comparator creating code from cmp_item_row::store_value().
It was responsible for non-temporal data types.
3. Removing find_date_time_item(). It's not needed any more.
All ROW-element data types are now covered by
cmp_item_row::prepare_comparators().
4. Adding a helper method Item_args::alloc_and_extract_row_elements()
to extract elements from an array of ROW-type Items, from the given
position. Using this method to collect elements from the i-th
position and further pass them to
Type_handler_hybrid_field_type::aggregate_for_comparison().
5. Moving the call for alloc_comparators() inside
cmp_item_row::prepare_comparators(). This helps
to call prepare_comparators() for ROW elements recursively
(if elements appear to be ROWs again).
Moving alloc_comparators() from "public" to "private".
